I am an ex-pro piercer, although if Dextra sees this thread, get her attention, she's more experienced than I (she trained me as a piercer about... 11 years ago).
The heart infection risk is there. It is the same risk as having bad dentistry. If you have an infection in your mouth, nasty stuff can get into your bloodstream and from there to your heart. It is also incredibly unlikely as the piercing heals over pretty quickly, making it so you don't have that acess to the bloodstream. A root canal is more dangerous in this sense. It has nothing to do with saliva corroding the jewelry, btw.
The brain abcess was a one-in-a-million. I've talked to the girl in question, as has Denise, and she got pierced again after. ANY damage to your skin can lead to something this weird. Just goes to show how important the skin is as our first line of defense.
As for speaking. Some people can talk fine with one, others get slight lisps, others get serious speach impediments. The trick here is a good piercer who knows good palcement, puts the right jewelry in, and can tell you whether your tongue is particularly short or has an extended web, both of which can lead to problems speech-wise.
as a footnote, I don't encourage the navel piercing that Joe brought up instead. Navel piercings are effectively 'surface piercings', which means the body has a higher chance of just pushing the piercing out. It is also in... your navel. Which gets dirty and sweaty. It doesn't heal quickly and is prone to infection. Worse, it sits somewhere where you will constantly be putting pressure on it (waistband of jeans, just bending forward when sitting down, etc). Again, trouble.
That, and most places won't allow you to wear one while working.
Actually, most places will never notice unless you make a habit of screaming or yelling or ywawning without putting your hand over your mouth. The issue is the first week, when your tongue most likely will be swollen, giving you a speech impediment. Most people speak just about normally after that first week. Denise / Dextra has 2 tongue piercings and they don't interfere with her speech AT ALL, nor her employability.
The actual piercing does not hurt much. I found my ear piercings to be more painful in honesty... Thinking over it, I can't think of a piercing I've had that hurt less than my tongue (maybe my eyebrow). It's the first day of swelling that 'hurts' - and that is a misnomer, it is actually VERY tender, so eating hurts, and talking hurts, and so on. So I recommend shutting up and drinking lots of milkshakes.

Oral piercings heal very quickly.
Now... long term.
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Dental Damage: Yes. It happens. More to some than others. I didn't have any damage to my teeth proper in the 9 years I wore mine. But I -DID- have some erosion of the gumline over the roots of my lower teeth, inside front. Denise has had -NO- dental damage, gum erosion or soft-palate damage from hers. But if you look around, you will find people who have had worse experiences than the two of us. Chipping occurs sometimes if the barbell is too long and you bite on it. Tapping of the barbell against teeth can result in some or much enamel damage... your millage may vary.